This invention relates to a process for hot-rolling a carbon or stainless steel that has already been hot-rolled by a tandem or other process. More specifically, it is related to a process for using a tandem hot-rolling mill for reducing the thickness of a carbon or stainless steel by reheating a coil of hot-rolled steel and then rerolling the coil in the same tandem rolls used to initially roll the coil.
Hot-rolling is the most widely used method to shape a cast slab into a coil form. No other process is as effective or as efficient as hot-rolling in reducing slabs of, say, 0.8" thick to 0.1". However, one of its disadvantages is scale formation during or after rolling due to the high temperature operation. As a result, the surface appearance and gage uniformity of hot-rolled products are inferior to those of cold-rolled products.
Further, it is impractical to roll a slab beyond a product width-to-thickness ratio of 1000:1. This limit is primarily due to the non-uniform temperatures existing across the width of a coil generated by the long slab processing sequence, and the excessive roll forces required for further reduction.
In spite of these disadvantages, the commercial and industrial value of thin gage, hot-rolled products has been recognized for some time. Many high strength carbon and stainless steels are work-hardened so rapidly during cold-rolling that either a very high rolling force or a special roll arrangement is required for processing, such as expensive Sendzimir mills, for obtaining lighter gage stainless steel. One way to tolerate the work-hardening behavior is to start with a thin gage hot-rolled steel, minimizing the subsequent cold reduction requirement.
Attempts have been made to manufacture hot-rolled thin gage steel by changing the roll arrangement, such as using planetary and cluster mills. In principle, the smaller the working roll diameter is, the more reduction it can achieve. Hence, most roll mill designers are working on the support system of the small-size work roll. However, little progress has been made in developing low-cost, hot-rolling technology for achieving thin gage steel.
Mannesmann Demag Huttentechnik, a supplier to the metallurgical industry, and Finarvedi in Cremona, Italy, an Italian company in the field of producing special steels, has developed the I.S.P. (Inline Strip Production) process. This process produces hot-rolled strips.
In this process, thin slabs are produced, induction heated, rolled into an intermediate gage, and then coiled onto a mandrel in a special furnace. This special furnace allows two strips to be housed in such a way that while one strip is coiled onto one mandrel, a second strip on a second mandrel is uncoiled and sent to the tandem hot-rolling stands without losing temperature. In this way, relatively thin gage, hot-rolled steels can be obtained. This development is essentially a combination of the Steckel mill concept and tandem rolling mills. The Steckel mill is widely used by the stainless and alloy steel manufacturers who do not have a tandem rolling mill.
Some of the disadvantages of the Demag/Arvedi I.S.P. process are the extra capital investment required for installing an additional rolling mill and the limit of the gage that it can produce.